The present invention relates generally to an optical data format converter that converts non-return-to-zero (NRZ) optical signals to return-to-zero (RZ) optical signals and vice versa.
Two types of optical data format are typically used in optical communications systems: (1) non-return-to-zero (NRZ) format and (2) return-to-zero (RZ) format. In NRZ format, a xe2x80x981xe2x80x99 bit in an optical data stream is encoded as an optical data signal present for substantially all of a period of the signal, where a period is an arbitrary length of time allocated to each bit in the data stream. In RZ format, a xe2x80x981xe2x80x99 bit is encoded as a small width pulse within the period of the signal (i.e., the optical signal goes to xe2x80x981xe2x80x99 and then returns to xe2x80x980xe2x80x99). It may be desirable to use different formats within a single optical system or in communicating optical systems and therefore a method for converting one format to another is needed.
For example, the NRZ data format has typically been the industry choice for wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) because: (1) it requires lower bandwidth in the electronic states of the receiver; (2) it has a high tolerance to timing jitter; and (3) stable continuous wave (CW) distributed feedback (DFB) lasers and high-quality LiNbO3 modulators needed for NRZ data format are readily commercially available. More than 100 wavelengths can be concurrently transmitted between nodes of an optical network using NRZ data format in a WDM system. However, due to a much longer cross-wavelength interaction time, compared to RZ format, nonlinear distortions produced by self-phasing modulation, cross phase modulation, four wave mixing, and stimulated Raman and Baillouin scattering (SRS and SBS) introduce tremendous challenges to these systems. Also, NRZ formatted data requires under-compensation, a technique used to pre-shape pulses to compensate for pulse spreading due to linear chromatic dispersion. The degree of under-compensation depends on signal power, transmission distances and amplifier spacing.
When fewer channels are implemented in a WDM network system, RZ data format increases the individual channel bit rate. The RZ formatted data pulses: (1) suppress SBS significantly because of much wider spectrum (i.e., because the wavelengths of the channels are not typically packed as close together as NRZ formatted data); (2) introduce less nonlinear distortions due to reduced crossing wavelength interaction time; (3) have better receiver sensitivity (i.e., weaker signals can be accurately detected for RZ data compared to NRZ data); (4) have fewer dependencies on signal launching power and transmission distance for dispersion compensation; and (5) can be used for soliton communications. RZ format has proven to be superior to NRZ format especially at bit rates of 10 to 20 Gb/s or even 40 Gb/s per channel. Therefore, in newly designed and installed network systems where dispersion is well managed, RZ data format becomes more attractive. On the other hand, for fiber systems installed in the 1970""s and 1980""s, NRZ format is usually a better choice; most fiber systems installed at that time are single mode fiber that causes no dispersion near 1.3 microns, a wavelength commonly used for NRZ data. Dispersion is not however compensated well in these fiber systems near 1.5 microns, a wavelength commonly used for RZ data. In cases where dispersion induced distortions dominate, NRZ format is a better choice.
RZ data format is also the format of choice for Optical Time Division Multiplexed (OTDM) systems, which bit interleave very short (e.g., picosecond) RZ pulses. OTDM is often used in high speed LAN and computer interconnects.
Selection of a particular format depends on specific parameters in transmission links or network systems. For example, for WDM, an optimal pulse width or data format exists for a specific overall dispersion, launch power, wavelength spacing bit rate, etc. A device that could adjust the pulse width of an optical signal would be a powerful tool to balance design trade-offs; for example, such a device could (1) convert NRZ format signals used in one part of a WDM network to RZ format used in another part of the network, (2) interface a long haul soliton transmission system with an existing NRZ transmission system, (3) interface an OTDM LAN to a DWDM network. Additionally, such a device could improve interoperability by maintaining optical transparency.
It is therefore an object of the present invention to provide a device and method for converting RZ optical pulses to NRZ format.
It is a further object of the present invention to provide a device and method for converting NRZ optical pulses to RZ format.
It is yet another object of the present invention to provide a device and method for lengthening and compressing the pulse width of an optical signal.
It is still another object of the present invention to provide a device for converting the wavelength of an optical signal to a different wavelength.
Briefly, an optical data format converter in accordance with the present invention uses a Terahertz Optical Asymmetric Demultiplexer (TOAD) to increase or decrease the duty cycle of an optical signal. In an embodiment that increases the duty cycle, such as converting RZ pulses to NRZ format, the optical data is injected at the clock input port of the TOAD and a continuous wave (CW) laser feeds the data input port. A stretched copy of the input signal will appear at the output port of the TOAD, with the output pulse width determined by the TOAD sampling, or switching, window. Also, if the input signal is at a different frequency than the CW laser, then by using a narrow linewidth CW laser at the TOAD""s data input port and placing a narrow band optical filter at the output, the format converter can also reduce the amount of noise in the input signal and transmit a cleaner signal to a receiver. In this case, the converter will also act as a wavelength converter, converting the wavelength of the input data signal to the wavelength of the CW laser. An input RZ signal and an output NRZ signal is illustrated in FIG. 1 (i.e., the format converter can increased the RZ signal duty cycle to about 100% such that data format is converted to NRZ).
In an embodiment that decreases the duty cycle, such as converting NRZ pulses to RZ format, the optical data is injected at the data input port of the TOAD and a pulsed control signal is injected at the clock input port. The sampling window is selected to be smaller than that period of the NRZ signal. Only the portion of the NRZ signal that overlaps the window appears at the output of the TOAD and thus the duty cycle is decreased.
Various implementations of the TOAD exist, such as the Sagnac TOAD, the Mach-Zehnder TOAD, and the Michelson TOAD. Each of the various TOAD implementations can be used in a format converter in accordance with the present invention and invention is intended to embrace all TOAD implementations.